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IS 15656 (2006) : Hazard Identification and Risk 
Analysis — Code Of Practice. ICS 13.100 



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Invent a New India Using Knowledge 



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"Knowledge is such a treasure which cannot be stolen" 





18 15656:2006 



Indian Standard 

HAZARD IDENTIFICATION AND 
RISK ANALYSIS — CODE OF PRACTICE 



ICS 13.100 



©BIS 2006 

BUREAU OF INDIAN STANDARDS 

MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG 
NEW DELHI 110002 

May 2006 Price Group 9 



Occupational Safety and Health and Chemical Hazards Sectional Committee, CHD 8 



FOREWORD 

This Indian Standard was adopted by the Bureau of Indian Standards after the draft finalized by Occupational Safety 
and Health and Chemical Hazards Sectional Committee had been approved by the Chemical Division Council, 

With the progressive advances in technology, the continuing trend towards larger and more highly integrated production 
units, and the ever-increasing demand by governmental and public bodies for improved safety and environmental 
standards, hitherto conventional methods of design based on established principles and Codes of practice are no longer 
adequate in themselves for ensuring acceptable standard of safety in process industry. As a preventive measure of 
minimizing the chance of accident to occur in hazardous installations and thereby reducing the possibility of injury, 
loss of material and degradation of the environment, it is necessary to use more searching and systematic methods for 
risk control to supplement existing procedures. The inherent property of material used in the process and the processes 
themselves pose the potential hazard in any hazardous installation and a comprehensive risk assessment is needed for 
effective management of risk, which needs to be identified, assessed and eliminated or controlled. The techniques 
should be used fi-om the conception of a project and must be used periodically throughout the life of an installation to 
the point of decommissioning. The assessment of hazards is carried out by combination of hazard analysis, consequence 
analysis and probability calculations. 

Prevention of human and property losses is integral to the operation and management of chemical process plants. This 
may be achieved through the selection of a technology that is inherently safe. Alternatively safety of plant design and/ 
or operation can be audited by the application of hazard identification and risk analysis techniques, and adopting 
measures suggested by the analysis. The latter approach constitutes Quantitative Risk Analysis (QRA). 

This Code of practice is intended for safety professionals and engineers in the areas of chemical plant safety to upgrade 
safety performance of the plants and covers the methods of identifying, assessing and reducing hazards including 
evaluation and selection of methods for particular applications. A few useful techniques are elaborated with worked out 
examples. 

In the formulation of this standard, considerable assistance has been derived fi*om the following publications: 

a) Guidelines for Hazard Evaluation Procedures, Centre for Chemical Process Safety, American Institute of 
Chemical Engineers, 1992. 

b) Guidelines for Chemical Process Quantitative Risk Analysis, Centre for Chemical Process Safety, American 
Institute of Chemical Engineers, 2000. 

c) TheMondlndex, Imperial Chemical Industries (ICI) PLC, 1993. 

d) DOW*s Fire and Explosion Index - Hazard Classification Guide, American Institute of Chemical Engineers, 
1994. 

e) DOW's Chemical Exposure Index Guide, American Institute of Chemical Engineers, 1994. 

f) Methods for Determination of Possible Damage to People and Objects Resulting from Release of Hazardous 
Materials — Committee for the Prevention of Disasters caused by Dangerous Substances, The Hague, 1992, 
TNO. 

g) Methods for Calculation of Physical Effects — Committee for the Prevention of Disasters caused by Dangerous 
Substances, The Hague, 1997, TNO. 

The composition of the technical committee responsible for formulating this standard is given at Annex G 

For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed 
or calculated, expressing the result of a test or analysis, shall be rounded off in accordance with IS 2 : 1960 'Rules for 
rounding off numerical values {revised)'. The number of significant places retained in the rounded off value shall be 
the same as that of the specified value in this standard. 



IS 15656 : 2006 



Indian Standard 

HAZARD IDENTIFICATION AND 
RISK ANALYSIS — CODE OF PRACTICE 



1 SCOPE 

This Code describes specific techniques to prevent human 
and property losses in the operation and managSment of 
process plant. The overall methodology presented in this 
Code allows systematic identification of hazards as well 
as quantification of the risks associated with the operation 
of process plants. Applied with due expertise and rigour 
the prescribed methodology can help the user understand 
the relative levels of hazards and risk potential in an 
installation. This aids the selection and prioritization of 
necessary strategies for accident prevention and limiting 
their consequences. Therefore, the Code can be used for 
improving plant safety performance as well as to reduce 
human and property losses. Risk analysis is a process that 
consists of a number of sequential steps as follows: 

a) Hazard Identification — Identifying sources of 
process accidents involving release of hazardous 
material in the atmosphere and the various ways 
(that is scenarios) they could occur. 

b) Consequence Assessment — Estimating the 
probable zone of impact of accidents as well as the 
scale and/or probability of damages with respect to 
human beings and plant equipment and other 
structures. 

c) Accident F/equency Assessment — Computation 
of the average likelihood of accidents. 

d) Risk Estimation — Combining accident 
consequence and frequency to obtain risk 
distribution within and beyond a process plant. 

This Code describes the essential nature of each of the 
above sequence of steps and describes a variety of 
techniques for identifying hazards and the quantification 
of accident consequence and the frequency towards the 

final risk estimation. 

o 

The Quantitative Risk Analysis (QRA) is most applicable 
and provides meaningful results when a plant is built, 
operated and maintained as per design intent and good 
engineering practices. 

2 TERMINOLOGY 

For the purpose of this Code, the following technical terms 
used are interpreted and understood as given below. 

2.1 Accident — A specific unplanned event or sequence 
of events that has undesirable consequences. 



2.2 Basic Event — A fault tree event that is sufficiently 
basic that no further development is necessary. 

2.3 Consequence — A measure of the expected effects 
of an incident. 

2.4 Explosion — A sudden release of energy characterized 
by accompaniment of a blast wave. 

2.5 External Event — An event caused by a natural hazard 
(earthquake, flood, etc) or man-induced events (aircraft 
crash, sabotage, etc). 

2.6 Fire — A process of combustion characterized by 
heat or smoke or flame or any combination of these. 



2.7 Frequency - 

per unit of time. 



- The number of occurrences of an event 



2.8 Hazard — A characteristic of the system/plant process 
that represents a potential for an accident causing damage 
to people, property or the environment. 

2.9 Initiating Event — The first event in an event 
sequence. 

2.10 Mitigation System — Equipment and/or procedures 
designed to respond to an accident event sequence by 
interfering with accident propagation and/or reducing the 
accident consequence. 

2.11 Probability — An expression for the likelihood of 
occurrence of an event or an event sequence during an 
interval of time or the likelihood of the success or failure 
of an event on test or on demand. 

2.12 Risk — A measure of potential economic loss or 
human injury in terms of the probability of the loss or 
injury occurring and the magnitude of the loss or injury if 
it occurs. 

2.13 Top Event — The unwanted event or incident at the 
top of a fauh tree that is traced downward to more basic 
failures using logic gates to determine its causes and 
likelihood 

2. 1 4 Worst Case Consequence — A conservative (high) 
estimate of the consequences of the most severe accident 
identified. 



1 



IS 15656 : 2006 



3 RISK ANALYSIS METHODOLOGY 

The flow chart for risk analysis is given in Fig. 1 

3.1 The terms in Fig. 1 are explained as follows. 



3.1.1 Goal 

Goal for carrying out risk analysis is required as a part of 
statutory requirement, emergency planning, etc. depending 
on the nature of industry. 



DEFINE THE GOAL (STATUTORY, 

EMERGENCY PLANNING 

CONSEQUENCE, ETC. 



LOCATION, LAYOUT. PROCESS 
PARAMETERS 



HAZARD IDENTIFICATION 



QUANTIFICATION OF 
HAZARD 



SELECT MOST CREDIBLE 
SCENARIO 




SELECT WORST CASE SCENARIO 



ESTIMATE CONSEQUENCES •- EMERGENCY PLAN 



ESTIMATE EFFECT OF DAMAGE 




ESTIMATE FREQUENCY OF 
OCCURRENCE 



YES 



-TendJ 



ESTIMATE RISK 



PRIORITIZE AND REDUCE RISK 



Fig. 1 Flow Chart for Risk Analysis 
2 



18 15656:2006 



3.1.2 Location, Layout, Process Parameters 

The information on plant location, the layout of equipment, 
the process conditions, etc, is required for the risk analysis. 

3.1.3 Hazard Identification 

Hazard identification is done by comparative and/or 
fundamental methods leading to qualitative or quantitative 
results. 

3.1.4 Quantification of Hazards 

The indices method for hazard identification can assess 
the hazard potential for the identified scenarios and can 
be used as a tool for screening. 

3.1.5 Select Most Credible Scenario 

The credible scenarios which can culminate into an 
accident out of several major and minor scenarios, possible 
for the release of material and energy. 

3.1.6 Select Worst-Case Scenario 

The incident, which has the highest potential to cause an 
accident of maximum damage, is selected for further 
analysis. 

3.1.7 Estimate Consequences 

The consequences of scenarios in the plant in the form of 
fire, explosion and toxic effects have to be estimated and 
presented. 

3.1.8 Estimate Frequency of Occurrence 

. The probability or frequency of its occurrence of any 
incident is to be found out by reliability analysis, which 
includes fault tree/event tree, etc. 

3.1.9 Estimate the Risk 

Risk is expressed as the product of frequency of an event 
and the magnitude of the consequences that result each 
time the event occurs. The calculated risk can be compared 
with national or international values. 

3.1.10 Prioritize and Reduce Risk 

Based on the estimated risk the contributing factors leading 
to events/accidents are analysed and prioritized in the risk 
analysis. 

4 STAGES OF PROCESS PLANT AND RISK 
ANALYSIS 

The life span of a process industry comprises a number of 
stages from conceptual to decommissioning. Each stage 
of a plant may have hazards, some general and some stage- 
specific. Hazard identification and risk analysis techniques 
that may be applied at different stages of a plant are given 
in Table 1. 



Table 1 Plant Stages vls-a-vls Hazard Identification 
and Hazard Analysis Techniques 



SI No. Project Stage 




Hazard Identification/ 








Hazard Analysis Techniques 


(1) 


(2) 




(3) 


i) 


Pre-design 


a) 


Hazard indices 






b) 


Preliminary hazard analysis 






c) 


What-if analysis 






d) 


Checklists 


ii) 


Design/Modification 


a) 


Process design checks and use of 
checklist 






b) 


HAZOP studies 






c) 


Failure modes and eflfects analysis 






d) 


Vv'hat-if analysis 






e) 


Fault tree analysis 









Event tree analysis 


iii) 


Construction 


a) 


Checklists 






b) 


What-if analysis 


iv) 


Commissioning 


a) 


Checklist 






b) 


Plant safety audits 






c) 


What-if analysis 


V) 


Operation and 


a) 


Plant safety audits 




maintenance 


b) 


What-if analysis 






c) 


Checklists 


vi) 


Decommissioning/ 


a) 


Checklists 




Shutdown 


b) 


What-if analysis 



5 HAZARD IDENTIFICATION AND HAZARD 
ANALYSIS 

A hazard is generally realised as a loss of containment of 
a hazardous material. The routes for such loss of 
containment can include release from pipe fittings 
containing liquid or gas, releases fi*om vents/relief and 
releases from vessel rupture. Adhering to good engineering 
practices alone may not be adequate for controlling plant 
hazards thus, a variety of techniques of hazard 
identification and probability of their occurrence have been 
developed for analysis of processes, systems and 
operations. 

The objective of hazard identification is to identify and 
evaluate the hazards and the unintended events, which 
could cause an accident. The first task usually is to identity 
the hazards that are inherent to the process and/or plant 
and then focus on the evaluation of the events, which could 
be associated with hazards. In hazard identification and 
quantification of probability of occurrence it is assumed 
that the plant will perform as designed in the absence of 
unintended events (component and material failures, 
human errors, external event, process unknown), which 
may affect the plant/process behaviour. 

5.1 Hazard Identification 

Formal hazard identification studies generate a list of 
failure cases. The list can usually be derived reliably by 



IS 15656 : 2006 



considering: (a) form in which chemicals are stored or 
processed, (b) nature of hazard it poses, and (c) quantity 
of the material contained. The hazard identification 
methods may be categorized as comparative methods and 
fundamental methods. These techniques are also described 
in A-2. 

5,1.1 Comparative Methods 

These techniques are based on hazard identification by 
comparing with standards. The various methods are 
check hst, safety audit, hazard indices and preliminary 
hazard analysis. 



5.1.1.1 Checklist 

Purpose 

Applicability 



For quick identification of hazards. 

In all phases — design construction, 
commissioning, operation and 
shutdown. 

Data required Checklist is prepared from prior 
experience/standard procedure/ 
manual/ knowledge of system or plant. 

Results Essentially qualitative in nature and 

leads to "yes-or-no" decision with 
respect to compliance with the standard 
procedure set forth. 

5.LL2 Safety audit 

Purpose For ensuring that procedures match 

design intent. 

Applicability In all phases of the plant and periodicity 
of review depending on the level of 
hazard. 

Data required Applicable codes and guides, plant flow 
sheet, P & 1 diagrams, start-up/ 
shutdown procedure, emergency 
control, injury report, testing and 
inspection report, material properties. 

Results Qualitative in nature — the inspection 

teams report deviation from design and 
planned procedures and recommends 
additional safety features. 

5. LI .3 Hazard indices 

Pui*pose For identifying relative hazards. 

Applicability In design and operation phase used as 
an early screening technique for fire/ 
explosion potential 

Data required Plot plan of a plant, process flow 
condition, Fire and Explosion Index 
Form, Risk Analysis Form, Worksheets. 

Results Relative quantitative ranking of plant 

process units based on degree of risk. 



5. 1 . 1 .4 Preliminary hazard analysis 

Purpose For early identification of hazards. 

Applicability In preliminary design phase to provide 
guidance for final design. 

Data required Plant design criteria, hazardous 
materials involved and major plant 
equipment. 

Results List of hazards (related to available 

design details) with recommendation to 
designers to aid hazard reduction. 

5. 1 .2 Fundamental Methods 

These techniques are a structured way of stimulating a 
group of people to apply foresight along with their 
knowledge to the task of identifying the hazards mainly 
by raising a series of questions. These methods have the 
advantage that they can be used whether or not the Codes 
of practice are available for a particular process. Three 
main techniques are available in this family of methods 
that is What-if Analysis, Failure Modes and Effects 
Analysis, (FMEA) and Hazard and Operability Study 
(HAZOP). 

5.1.2.1 What-if analysis 

Purpose Identifying possible event sequences 

related to hazards. 

Applicability During plant changes, development 
stage or at pre start-up stage. 

Data required Detailed documentation of the plant, the 
process and the operating procedure. 

Results Tabular listing of accident scenarios, ' 

their consequences and possible risk 
reduction methods. 

5.1.2.2 Failure modes and effects analysis 

Purpose Identifying equipment failure modes and 

their effects 

Applicability In design, construction and operation 
phases, useful for plant modification. 

Data required Knowledge of equipment/system/plant 
functions. 

Results Qualitative in nature and includes worst- 

case estimate of consequence resulting 
from failure of equipment. 

5. 1 .2.3 Hazard and operability study 

Purpose Identifying hazard and operability 

problem. 

Application Optimal when applied to a new/ 

modified plant where the design is 
nearly firm. 

Data required Detailed process description, detailed 



IS 15656 : 2006 



P&I drawing and operating procedure 
for batch process. 

Results Identification of hazards and operating 

problems, recommends change in 
design, procedure and further study. 

5.2 Hazard Analysis 

The principle techniques are fault tree analysis (FTA) and 
event tree analysis (ETA). These techniques are also 
described in A-3. 

5.2.1 Fault Tree Analysis 

Purpose Identifying how basic events lead to an 

accident event. 

Applicability In design and operation phases of the 
plant to uncover the failure modes. 

Data required Knowledge of plant/system function, 
plant/system failure modes and effects 
on plant/system. 

Results Listing of set of equipment or operator 

failures that can result in specific 
accidents. 

5.2.2 Event Tree Analysis 

Purpose Identifying the event sequences from, 

initiating event to accident scenarios. 

Applicability In design/operating plants to assess 
adequacy of existing safety features. 

Data required Knowledge of initiating events and 
safety system/emergency procedure. 

Results Provides the event sequence that result 

in an accident following the occurrence 
of an initiating event. 

6 CONSEQUENCE ANALYSIS METHODOLOGIES 

All processes have a risk potential and in order to manage 
risks effectively, they must be estimated. Since risk is a 
combination of frequency and consequence, consequence 
(or impact) analysis is a necessary step in risk analysis. 
This section provides an overview of consequence and 
effect models commonly used in risk analysis. 

An accident begins with an incident, which usually 
results in loss of containment of material. The material 
may possess hazardous properties such as flammability, 
explosivity, toxicity, etc. Typical incidents might include 
the rupture of a pipeline, a hole in a tank or pipe, runaway 
reaction, external fire impinging on the vessel and 
heating it. 

Once the incident is defined source models are selected to 
describe how materials are discharged from the 
containment. Source models provide a description of the 



rate of discharge, the total quantity discharged, the duration 
of discharge, and the state of discharge, that is liquid, 
vapour or two-phase fiow. Evaporation models are 
subsequently used to calculate the rate at which the material 
becomes air-borne. 

Next a dispersion model is used to describe how the 
material is transported downwind and dispersed to 
specified concentration levels. For flammable releases, fire 
and explosion models convert the source model 
information on the release into energy hazard such as 
thermal radiation flux and explosion overpressures. 
Finally effect models convert these incident specific results 
into effects on people and structures. Environmental 
impacts could also be considered but these are beyond the 
scope of the present Code. 

In this Code a brief introduction to the methods of 
consequence analysis is provided. Annex F shows the steps 
to be followed in consequence analysis. These models are 
also described in A-4. 

6,1 Source Models 

Source models are used to quantitatively define the loss 
of containment scenario by estimating the discharge rate, 
total quantity released, release duration, extent of flash 
and evaporation from a liquid pool and aerosol formation 
and conversion of source term outputs to concentration 
fields. 

6. LI Discharge Rate Models 

Purpose Evaluation of discharge of material. 

Applicability First stage in developing the 
consequence estimates. 

Data required a) Physical condition of storage. 

b) Phase at discharge. 

c) Path of the discharge (hole size). 

Results a) Discharge rate of the gas/liquid/ 

two-phase flow. 

b) Durafion of release. 

c) Phase change during release. 

6.L2 Flash and Evaporation Models 

Purpose Estimation of the total vapour. 

Applicability During spillage of liquid on surface 
because of loss of containment. 

Data required a) Heat capacity, latent heat, boiling 
point of liquid. 

b) Leak rate, pool area, wind velocity, 
temperature. 

c) Vapour pressure, mass transfer 
coefficient. 

d) Viscosity, density, a turbulent 
friction coefficient. 



IS 15656 : 2006 



Results a) Amount of vapour from a liquid 

discharge. 

b) Time dependent mass rate of boiling. 

c) Radius or radial spread velocity of 
the pool. 

6.1.3 Dispersion Models 

Accurate prediction of the atmospheric dispersion of 
vapours is central to consequence analysis. Typically, the 
dispersion calculations provide an estimate of the 
geographical area affected and the average vapour 
concentrations expected. The simplest calculations require 
an estimate of the released rate of the gas, the atmospheric 
conditions, surface roughness, temperature, pressure and 
release diameter. Two types of dispersion models are 
usually considered: 

a) Positively buoyant or neutrally buoyant, and 

b) Negatively buoyant or dense gas. 

The dispersion of gases that are lighter than or equal to 
the density of dispersing medium are considered as 
positively buoyant and the gases with higher density at 
the point of dispersion is considered as negatively buoyant 
or dense gas. The dispersion is further categorized into 
puff model that is, instantaneous release or plume model 
that is continuous release or time varying continuous 
release, 

A large number of parameters affect the dispersion of gases. 
These include atmospheric stability, wind speed, local 
terrain effects, height of the release above the ground, 
release geometry, that is, point, line or area source, 
momentum of the material released and the buoyancy of 
the material released. 

Annex C gives the meteorological conditions defining the 
Pasquill-Gifford Stability Classes denoted by letters A to 
F, which correlate to wind-speed and cloud cover. The 
stability is commonly defined in terms of atmospheric 
vertical temperature gradient. For local application, the 
wind speed and cloud cover should be taken from 
meteorological records. For practical purpose two stability 
conditions given below can be used to find the dispersion 
pattern: 

Normal: 'D' at wind velocity of 5 m/s (Windy day time 
condition), and Extreme calm: 'F' at wind velocity of 
2 m/s (Still night-time condition). 

Annex D gives the terrain characteristics that affect the 
mixing of the released gas and air as they flow over the 
ground; thus the dispersion over a lake would be different 
from that over a tall building. Values of the surface 
roughness vary from 10 m for highly urban area to 
0.000 1 m over sea. For most practical purposes flat rural 
terrain (Few trees, long grass, fairly level grass plains) 
with surface roughness value of 0.1 is used. 



As the release height increases, the ground level 
concentration decreases since the resulting plume has more 
distance to mix with fresh air prior to contacting the ground. 

6.1.3.1 Positively buoyant or neutral dispersion model 

Purpose Prediction of average concentration — 

time profile. 

Applicability Used in prediction of atmospheric 
dispersion of lighter gases than air. 

Data required Discharge rate, release duration, 
stability class, wind speed, location, 
averaging time, roughness factor. 

Results Downwind concentration, area affected, 

duration of exposure. 

6.1.3.2 Negatively buoyant or dense gas model 

Purpose Prediction of average concentration — 

time profile. 

Applicability Used in prediction of atmospheric 
dispersion denser than air. 

Data required Discharge rate, release duration, density 
of air, density of fluid, location. 

Results Downwind concentration, area affected, 

duration of exposure. 

6.2 Fires and Explosions Models 

These models are used only when the material released is 
flammable and the yapour cloud concentration is within 
the flammable range. The various types of fire and 
explosion models are: 

a) Pool fires, 

b) Jet fires, 

c) Flash fires, 

d) Vapour cloud explosions, 

e) Boiling liquid expanding vapour explosions 
(BLEVE),and 

f) Physical explosions. 

6.2.1 Pool Fire Model 

Purpose Calculation of thermal radiation. 

Applicability Fire resulting from burning of pools of 
flammable liquid spilled. 

Data required Quantity, pool diameter, heat of 
combustion and vaporization, density of 
air, temperature, view factor, etc. 

Results Thermal radiation flux at a distance. 

6.2.2 Jet Fire Model 

Purpose Calculation of thermal radiation. 

Applicability Fire resulting from combustion of 
material as it is being released from 



IS 15656 : 2006 



pressurized process unit. 

Data required Flow rate, hole diameter, heat of 
combustion and vaporization, density of 
fluid, temperature, view factor, etc. 

Results Thermal radiation flux at a distance. 

6.2.3 Flash Fire Model 

Purpose Calculation of thermal radiation. 

Applicability Fire resulting from non-explosive 
combustion of a vapour cloud. 

Data required Material released, dispersion 
coefficients, flame emissivity, view 
factor, atmospheric attenuation. 

Results Thermal radiation flux at a distance. 

6.2.4 Vapour Cloud Explosion Model 

Purpose Calculation of overpressure. 

Applicability Explosion of a flammable cloud formed 
due to release/flashes to vapour. 

Data required Mass of flammable material in vapour 
cloud, heat of combustion of material, 
etc. 

Results Overpressure at a distance. 

6.2.5 Boiling Liquid Expanding Vapour Explosion 
(BLEVE) Model 

Purpose Calculation of thermal radiation. 

Applicability Release of a large mass of pressurized 
superheated liquid to the atmosphere. 

Data required Mass involved in fire ball, radiative 
fraction of heat of combustion, heat of 
combustion for unit mass, atmospheric 
transmissivity. 

Results Thermal radiation flux from the surface 

of fireball. 

6.2.6 Physical Explosion Model 

Purpose Calculation of missile damage 

Applicability Vessel rupture resulting in release of 
stored energy producing a shock wave. 

Data required Pressure, volume, heat capacity, mass 
of container, ratio of heat capacities, 
temperature. 

Results Overpressure at a distance, fragment 

size and velocity 

6.3 Effect Model 

This model is described in A-5. 

Applicability Method of assessing property damage 
and human injury/fatality due to: 



a) thermal radiation. 

b) overpressure. 

c) toxic exposure. 

Data required In the Probit function Pv^ a + b\n V 
the causative factor V in the Probit 
Equation varies as follows; 

a) Fire: Pr = a + 6 In (/ V% t is duration 
of exposure and / is thermal intensity 

b) Explosion: Pr = a + ^ In (Ps), where 
Ps is the peak over pressure 

c) Toxicity: Px^a + bXn (C"tc), where 
C = concentration in ppm by 
volume, tc = exposure time, in 
minutes and n = constant. 

The constants a and b in the probit 
equation are calculated from the 
experimental data and are available in 
Methods for determination of possible 
damage to people and objects resulting 
from release of hazardous materials 
[^ee Foreword (f)]. 

Results The percent of fatality or the percent of 

damage to equipment. 

7 RISK CALCULATION 

7.1 Risk can be defined as a measure of economic loss, 
human injury or environmental damage both in terms of 
likelihood and magnitude of loss, injury or damage. In 
this document only the property damage, that is, economic 
loss and human loss have been considered. Risk is 
expressed as the product of frequency of an event and the 
magnitude of the consequences that result each time the 
event occurs. The mathematical expression for risk is: 

R = FC 
where 

R = risk (loss or injury per year); 

F = frequency (event per year); and 

C = consequence (loss or injury per event). 

7.2 In many cases the hazard cannot be completely 
eliminated though the probability of occurrence can be 
reduced with addition of safety measures and at a financial 
cost. 

7.3 The basic approach for estimating frequency has been 
discussed in 5.2. 

7.4 The consequence in terms of deaths/year or in terms 
of monetary loss per year can be estimated by the methods 
of consequence analysis described in 6. 

7.5 Risk Criteria 

Risk criteria are the acceptable levels of risk that can be 
tolerated under a particular situation. In many countries 



IS 15656 : 2006 



the acceptable risk criteria has been defined for industrial 
installations and are shown in Annex E. These criteria are 
yet to be defined in the Indian context, but values employed 
in other countries can be used for comparison. 

8 GUIDELINES FOR APPLICATION OF RISK 
ANALYSIS TECHNIQUES 

This Code essentially outlines the various approaches and 
techniques that may be used during the risk analysis of a 
process plant. This concluding section enumerates some 
of the critical features of the methodology of risk analysis 
so as to aid the prospective users apply the Code most 
effectively: 

a) While undertaking a risk analysis, careful 
consideration of the various possible approaches/ 
techniques is necessary, since each have their 
individual strengths and limitations. 

b) The method of risk analysis requires realistic 
accident scenario assumptions as well as 
comprehensive plant operational information and, 
in particular, reliable data pertaining to component/ 
system failure frequencies, human error rates, etc. 
In the event of any uncertainties relating to the 
relevant information and data, the use of experience 
and judgment would be critical to obtaining risk 
estimates that provide reliable support to subsequent 
decision-making, 

c) All assumptions applied during a risk analysis 
exercise need be documented with clarity, so as to 
enable better comparison and communication. 



d) In specific instances, the risk analysis method may 
require consideration of the external events as 
probable causative factors in large-scale hazardous 
chemical releases. 

e) Wherever feasible the risk analysis for a process 
plant should incorporate possible environmental 
consequences as well as possible human health 
effects that are immediate and/or delayed. 

f) Risk analysis need be undertaken newly in the event 
of any major changes introduced in the plant 
configuration. It must also be updated periodically 
whenever improved plant operational information 
and equipment/human failure data becomes 
available. Further, it is advisable to improve risk 
calculations using newer analytical methods as and 
when they are developed. 

With the techniques used for the analysis large number of 
results based on numbers of accident scenarios used, the 
various weather classes chosen, the assumptions in 
calculating each cases would be available. But finally it is 
very important to summarize all the results in one format 
providing clearly what factor appear to be important in 
overall analysis. A format has to be chosen for presenting 
the results of the analysis and acceptability is to be 
established either in terms of 'risk criteria' or 'distance 
under consideration which face the consequence' or '% 
damage up to a distance under consideration'. 

One typical format for reporting the analysis is given in 
Annex B. 



IS 15656 : 2006 



ANNEX A 
{Clauses 5 A^ 5,2 and 6) 

DETAILS OF CHEMICAL PROCESS RISK ANALYSIS METHODS 



A-1 HAZARD IDENTIFICATION AND RISK 
ANALYSIS SEQUENCE 

The purpose of hazard identification and risk analysis is 
to identify possible accidents and estimate their frequency 
and consequences. Conceivably the initiating event could 
be the only event but usually it is not and as a matter of 
fact there a number of events between the initiating event 
and the consequence and these events are the responses 
of the systems and the operators. Different responses to 
the same initiating event w^ill often lead to different 
accident sequences with varying magnitude of 
consequences. 

While identifying the hazard(s) a filtering process is carried 
and only portions with potential risk are involved for risk 
analysis. Hazard is not considered for further analysis, if 
(a) it is unrealisable, and (b) if it is not very significant. In 
many cases, once the hazard has been identified the 
solution is obvious. In some more cases the solution is 
obtained from experience. In many other cases it is taken 
care of by Codes of practice or statutory requirement. 



A-2 HAZARD IDENTIFICATION 
QUANTIFICATION 



AND 



A-2.1 Checklist 

These are simple and quick means of applying the 
experience to designs or situations to ensure that the 
features appearing in the list are not overlooked. Checklists 
are used to indicate compliance with the standard 
procedure. It is intended for standard evaluation of plant 
hazards and a convenient means of communicating the 
minimal acceptable level of hazard evaluation that is 
required for any job generally leading to 'yes-or-no' 
situation. 

The checklist is frequently a form for approval by various 
staff and management functions before a project can move 
from one stage to the next. It serves both as a means of 
communication and as a form of control and can highlight 
a lack of basic information or a situation that requires a 
detailed evaluation. 

Checklists are qualitative in nature; limited to the 
experience base of the author of the checklist, hence, 
should be audited and updated regularly. It is a widely 
used basic safety tool and can be applied at any stage of a 
project or plant development. Accordingly it is named as 
Process checklist. System checklist, Design checklist, etc. 



A process or system checklist can be applied to evaluating 
equipment, material, or procedures and can be used during 
any stage of a project to guide the user through common 
hazards by using standard procedures. 

A-2.2 Safety Audit 

It is an intensive plant inspection intended to identify the 
plant conditions or operating procedures that could lead 
to accidents or significant losses of life and property. It is 
used to ensure that the implemented safety/risk 
management programs meet the original expectations and 
standards. It is also called 'Safety review', 'Process 
review', and 'Loss prevention review'. In essence, safety 
audit is a critical appraisal of effectiveness of the existing 
safety programme in a plant. 

The review looks for major hazardous situation and brings 
out the areas that need improvement. The steps for the 
identification process are: 

a) Obtaining response from plant on a pre-audit 
quesfionnaire; 

b) Preparation of checklist, inspection and interview 
plant personnel; and 

c) Preparation of safety audit report in the form of 
recommendation. 

The results are qualitative in nature. The review seeks to 
identify inadequacy in design, operating procedures that 
need to be revised and to evaluate the adequacy of 
equipment maintenance or replacement. Assigning grades 
for effectiveness of safety management of the plant in the 
areas such as organization, operating procedures, 
monitoring, maintenance, etc is possible, a score card can 
be prepared to get an appraisal of safety status of plant. 

While this technique is most commonly applied to 
operating plants it is equally applicable to pilot plants, 
storage facilities or support fiinctions. 

The periodicity of such studies depends on the risk 
involved in the process and the commitment of the 
management. It usually varies from once in a year to one 
in seven years. 

A-2,3 Hazard kid ices 

Hazard indices can be used for relative ranking of process 
plants from the point of view of their hazard potentials. 
The most well known techniques are: 'DOW fire and 
explosion index', 'Mond fire. Explosion and toxicity index' 



IS 15656 : 2006 



and 'Chemical exposure index'. All these methods provide 
a direct and easy approach to a relative ranking of the 
risks in a process plant. The methods assign penalties and 
credits based on plant features. Penalties are assigned to 
process materials and conditions that can contribute to an 



accident. Credits are assigned to plant safety features that 
can mitigate the effects of an incident. These penalties 
and credits are combined to derive an index that is relative 
ranking of the plant risk. The following chart describes 
the use of such indices: 



Divide plant into units 



Assess unit 



Identify the significant material and calculate its material factor 



Use Mond form and manual to allocate penalty factors for: 

a) Special material hazard, 

b) General process hazard, 

c) Special process hazards, 

d) Quantity hazards, 

e) Layout hazards, and 

f) Acute health hazards. 



Calculate Indices for: 

a) Equivalent DOW, 

b) Fire, 

c) Internal explosion, 

d) Aerial explosion, and 

e) Hazard rating. 



If ratings are high then review input data, refine where possible and try alternatives 



Use Mond form and manuals to allocate credit factors for: 

a) Containment hazard, 

b) Process control, 

c) Safety attitude, 

d) Fire protection, 

e) Material isolation, and 

f) Fire fighting. 



Calculate offset indices for: 

a) Fire, 

b) Internal explosion, 

c) Aerial explosion, and 

d) Hazard rating. 



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The detailed methodology of using the Mond and the DOW 
indices for the hazard identification are not provided in 
this standard, for which users may look at different guides 
[see Foreword (c) and (d)]. 

The Chemical exposure index (CEI) method is a flirther 
developed technique derived from DOW F & E indices, 
useful for identification of hazards arising out of toxic 
chemicals present in a plant. It is also a tool to find out the 
requirement for further hazard assessment for such 
chemicals. 

It provides a simple method of rating the relative acute 
health hazards potential to people in the neighbourhood 
plants or communities from possible chemical release 
incidents. The methodology utilizes expression for 
estimating airborne quantity released from hazardous 
chemicals. The CEI system provides a method of ranking 
one hazard relative to other hazard but it is neither mtended 
to define a particular design as safe/unsafe nor to quantify/ 
detennine absolute measurement of risk. Flammability and 
explosion hazards are not included in this index. 

A-2.4 Preliminary Process Hazard Analysis 

It is used during the conceptual, early development, early 
design phase, of a plant. The method is intended for use 
only in the preliminary phase of plant development for 
cases where past experience provides little or no insight 
into potential safety problems, for example, a new plant 
with new process. Early identification of most of the 
hazards could be possible resulting in effective saving in 
cost that could otherwise result from major plant redesigns 
if hazards are discovered at a later stage. It is very useful 
for 'site selection'. It does not preclude the need for further 
hazard assessment; instead it is a precursor to subsequent 
hazard analysis. Items for consideration consist of 
meticulous preparation of a list of hazards: 

a) Raw materials, intermediates, by-products, final 
products; 

b) Plant equipment (high pressure systems); 

c) Interface among system components (material 
interactions, fire); 

d) Environment (earthquake, vibration, extreme 
temperature); and 

e) Operations (test maintenance and emergency 
procedure) Safety equipment. 

Example: 

Toxic gas 'A' is one of the components used in process; 
causes for the dangers: 

a) The hazards due to storing the gas; 

b) Hazards from the excess gas after the use; 

c) Lines supplying the gas *A' ; and 

d) Leakage during the receipt of the gas etc. 



The effects of these causes can be: 

a) Injury /Fatality to persons inside the plant or nearby 
areas, and 

b) Damage of property due to explosion. 

Safety measures/corrective actions provided to minimize 
effect: 

a) Whether less toxic material can be used; 

b) Minimizing the inventory for the storage of the 
material; 

c) Procedure for safe storage of the gas with enclosure 
system; 

d) Provision of plant warning system; 

e) Training for operators on properties, effect of 
material; and 

f) Informing neighboring localities about the toxic 
effect. 

The final results of the identification process can be 
recorded as: 

Hazard Causes Effects Preventive Measures 

A-2.5 Failure Modes and Effects Analysis 

The method is a tabulation of system/plant equipment, their 
failure modes, and each failure mode's effect on system/ 
plant. It is a description of how equipment fails (open, 
closed, on, off, leaks, etc) and the potential effects of each 
failure mode. The technique is oriented towards equipment 
rather than process parameters. FMEA identifies single 
failure modes that either directly result in or contribute 
significantly to an important accident. Human/operator 
errors are generally not examined in a FMEA; however, 
the effects of a mal-operation are usually described by an 
equipment failure mode. The technique is not efficient for 
idenfifying combinations of equipment failures that lead 
to accidents. A multidisciplinary team of professionals can 
perform FMEA. 

FMEA has following six main steps: 

a) Determining the level of resolution, 

b) Developing a consistent format, 

c) Defining the problem and the boundary conditions, 

d) Listing various failure modes, 

e) Each effects of the failure mode, and 

f) Completing the FMEA table. 

The level of resolution depends on the requirement of the 
plant, namely 'plant level', 'system level' or in other words 
whether the study is for a whole plant or a portion of plant 
or a particular system or individual equipment. Marking 
the portion of study on the drawing can indicate the 
physical system boundaries and stating the operating 



11 



18 15656:2006 



conditions at the interface. Identification of the equipment 
is necessary to distinguish between two or more similar 
equipment by any number and description of the equipment 
is required to give brief details about process or system. 

All the failure modes consistent with the equipment 
description are to be listed considering the equipment's 
normal operating conditions. 

Example of various failure modes of a normally operating 
pump is: 

a) Fails to open or fails to close when required, 

b) Transfers to a closed position, 

c) Valve body rupture, 

d) Leak of seal, and 

e) Leak of casing. 

The effects for each failure mode, for example, the effects 
of 'the fails to open condition for the pump' is: (a) loss of 
process fluid in a particular equipment, and (b) overheating 
of the equipment. The effect of pump seal leak is a spill in 
the area of the pump; if the fluid is flammable a fire could 
be expected, and so on. 

The analyst may also note the expected response of any 
applicable safety systems that could mitigate the effect. 

Example of the tabulated format may be: 
Plant 
System 

Boundary Condition 
Reference 

Equipment Description Failure modes Effect 

A-2,6 Hazard and Operability Study (HAZOP) 

The HAZOP study is made to identify hazards in a process 
plant and operability problems, which could compromise 
the plant's ability to achieve design intent. The approach 
taken is to form a multi-disciplinary team that works to 
identify hazards by searching for deviations from design 
intents. The following terms are used for the process for 
analysis: 

a) Intentions — Intention defines how the plant is 
expected to operate, 

b) Deviations — These are departures from intentions, 

c) Causes — These are reasons why deviations might 
occur, and 

d) Consequences — Results of deviations should they 
occur. 

The method uses guidewords, which are used to quantify 
or qualify the intention in order to guide and stimulate the 



hazard identification process. The guidewords are used to 
generate deviations from the design intent. The team then 
identifies cause and consequences of the deviations. 

HAZOP guidewords and their meanings: 

Guidewords Meaning 

No Negationof Design Intent 

Less Quantitative Decrease 

More Quantitative Increase 

Part of Qualitative Decrease 

As well as Qualitative Increase 

Reverse Logical Opposite to Intent 

Other than Complete Substitution 

The HAZOP study requires that the plant be examined for 
every line. The method applies all the guidewords in turn 
and outcome is recorded for the deviation with its causes 
and consequences. 

Example: 

a) For a particular line, 

b) Taking any guide word for example 'No', 

c) Deviation in process parameters, namely flow/ 
temperature, 

d) For each deviation the causes for such deviations, 

e) Consequences may be several C 1 , C2, C3, etc, and 

f) Measures to rectify the root cause for deviation. 

The tabulation of the results is made as follows: 



Guideword Deviation Causes Consequences Action 



A-2.7 What-If Analysis 

What-if analysis is used to conduct a thorough and 
systematic examination of a process or operation by asking 
questions that begms with What-If The questioning usually 
starts at the input to the process and follows the flow of 
the process. Alternately the questions can centre on a 
particular consequence category, for example, personnel 
safety or public safety. The findings are usually accident 
event sequences. Effective application of the technique 
requires in-depth experience of plant operation. 

Two fypes of boundaries that may be defined in a "What- 
If study are: (a) Consequence category being investigated, 
and (b) Physical system boundary. The consequence 
categories are mainly: (a) public risk, (b) worker risk, 
and (c) economic risk, for specific plant. The purpose of 
physical boundaries is to keep the investigating team 
focused on a particular portion of a plant in which 
consequence of concern could occur. The typical 
information required for What-if analysis is: 



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18 15656:2006 



a) Operating conditions, physical and chemical 
properties of materials, equipment description; 

b) Plot plan; 

c) Process and Instrumentation diagram of the plant 
including alarms monitoring devices, gauges etc; 

d) Responsibilities and the duties of the operating 
personnel, communication system etc; and 

e) Procedures for preventive maintenance, work 
permit system, for hazardous job, tackling 
emergency situations. 

The results are described in a chart, for example, for 
reaction of two substances A (toxic) and B. 



What-lf 



Hazard 



Recommendation 



Wrong delivery Not likely 
instead of B 

Actual product B Toxic gas may Concentration of B 
is in wrong be released . is to be checked 
concentration 



B is contami- 
nated 

Inlet Valve 
for B is closed 



Not likely 



Unreacted A will Alarm/shut-off for 
be released valve for the supply 

line for A 



A-3 QUANTIFICATION TECHNIQUES 

A-3.1 Fault Tree Analysis (FTA) 

It is a deductive technique that focuses on one particular 
accident event and provides a method for determining basic 
causes of that event. This method is used to identify 
combinations of equipment failures and human errors that 
can result in an accident or an initiating event. The solution 
of the fault tree is a list of the sets of equipment failures/ 
human error that are sufficient to result in the accident 
event of the interest. FTA allows the safety analyst to focus 
on preventive measures on these basic causes to reduce 
the probability of an accident. 

Essentially the fault tree is a graphical representation of 
the interrelationships between equipment failures and a 
specific accident. The equipment faults and failures that 
are described in a fault tree can be grouped into three 
classes, namely: 

a) Primary faults and failures — attributed to the 
equipment and not to any other external cause or 
condition. 

b) Secondary faults and failures — attributed to other 



external cause or condition. 

c) Commands faults and failures — attributed neither 
to equipment intended nor to any external cause 
but due to some source of incorrect command. 

There are four steps in performing the fault tree analysis: 

a) Problem definitions, 

b) Fault tree construction, 

c) Fauh tree solution (determining minimal cut sets), 
and 

d) Minimal cut set ranking. 

A-3. 1.1 Problem Definitions 

This consists of: (a) defining accident event — top event 
of the fault tree analysis, (b) defining analysis boundary 
including unallowed events, existing events, systems 
physical boundary, level of resolution, and other 
assumptions. 

A-3. 1.2 Fault Tree Construction 

It begins with the top event and proceeds level by level 
using symbols namely "Or" "And" etc. until all the fauh 
events have been developed to their basic contributing 
causes. 

A-3.L3 Fault Tree Solution 

The completed fault tree provides useful information by 
displaying the interactions of the equipment failures that 
could result in an accident. The matrix system of analysis 
gives the minimal cut sets, which are useful for ranking 
the ways in which accident may occur, and they allow 
quantification of the fault tree if appropriate failure data 
are available. 

A-3. 1 .4 Minimal Cut Set Ranking 

'Minimal cut set analysis' is mathematical technique for 
manipulating the logic structure of a fault tree to identify 
all combinations of basic events that result in occurrence 
of the top event. The ranking of minimal cut sets is the 
final step for the fauh tree analysis procedure. The basic 
events called the 'cut sets' are then reduced to identify 
those minimal cut sets which contain the minimal sets of 
events necessary and sufficient to cause the top event. 
Ranking may be based on number of basic events that are 
minimal cut set, for example, one event minimal cut is 
more important than two event minimal cut set; a two event 
minimal cut set is more important than three event minimal 
cut set and as on. This is because of the chance of 
occurrence of one event is more than that of two events to 
occur. Moreover, the human error is ranked at top, then 
the active equipment failure, then passive equipment 
failure. 



13 



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Example: 



NO LIGHT IN ROOM ON 
DEMAND T 



AND 



NO NATURAL LIGHT 

G1 



NO ARTIFICIAL LIGHT 

G2 



OR 





OR 




Fig. 2 Fault Tree for No Light in Room on Demand 



In Fig. 2 the causes Bl, B2, B3, B4 and B5 are the basic 
events, which can lead to Top event T, which is "No light 
in room on demand" and the mathematical expression for 
that top event is 

T = Gl xG2 

= (Bl+B2)x(B3 + B4 + B5) 

= B1B3 + B2B3 + B1B4 + B2B4 + B1B5 + B2B5 
(6 minimal cut sets) 

This indicates the occurrence of either of basic events 
B 1 or B2 along with occurrence of any of the basic events 
B3, B4 & B5 would lead to top event T {see Chart on 
page 15). 

In Fig. 3 the logic structure is mathematically transformed 
using Boolean Algebra into a minimal cut Fault tree. 

T - 01 xG2 

= (Bl +G3) + (B2+G4) 

- [Bl + (B3 X B4)] X (B2+B5+B6) 

which shows that any of the basic events B 1 -B6 should be 
in combinations as in the above expression to cause failure 
ofthe top event. 



A-3.2 Event Tree Analysis (ETA) 

ETA is a forward thinking process, begins with an initiating 
event and develops the following sequences of events that 
describe potential accidents accounting for: (i) successes, 
and (ii) failures ofthe available "safety function" as the 
accident progresses. The "safety function" includes 
operator response or safety system response to the initiating 
event. The general procedure for the event tree analysis 
has four major steps: 

a) Identifying an initiating event of interest, 

b) Identifying safety functions designed to deal with 
the identifying event, 

c) Construction ofthe event tree, and 

d) Results of accident event sequence. 

A-3.2. 1 Identifying an Initiating Event 

This identification of the event depends on the process 
involved and describes the system or equipment failure, 
human error or any other process upset that can result in 
other events. 

A-3.2. 2 Identifying Safety Functions 

The safety functions/safety systems available to mitigate 



14 



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DAMAGE TO REACTOR DUE TO HIGH 
PROCESS TEMPERATURE T 



1 




NO FLOW FROM 

QUENCH TANK 

G1 




1 



REACTOR INLET VALVE 
REMAINS OPEN 

G2 




QUENCH TANK 

VALVE DOES 

NOT OPEN 

G3 



OPERATOR 

FAILS TO 

CLOSE INLET 

VALVE 

04 




^INLETVALVE\ 

FAILS TO 

CLOSE 

B2 



OPERATOR FAILS TO 

OBEY ALARM AND 

CLOSE INLET VALVE 

B6 



Fig, 3 Fault Tree for Damage to Reactor Due to High Process Temperature 



the situation and deal with the identifying event include 
automatic shut down system, alarm system that alert the 
operator, operator action, containment method, etc. The 
analyst needs to identify all safety functions that can 
influence the sequence of events following the initiating 
event. The successes and the failures of the safety functions 
are accounted in the event tree. 

A-3.2.3 Construction of the Event Tree 

The event tree describes the chronological development 
of the accidents beginning with the 'initiating event'. 
Considering each safety functions to deal with the initiating 
event one nodal point is generated with the two alternatives 
(Al and A2) that is the 'success' and 'failure' of the safety 
system. At the first nodal point two alternatives are found 
to consider the second safety system/component to deal 
with the event. The success and failure of the second 
safety system also give branching to the two alternatives 
A3 and A4. 



A-3.2.4 Results of Accident Event Sequence 

The sequences of the constructed event tree represent a 
variety of outcomes that can follow the initiating event. 
One or more of the sequences may represent the safe 
recovery and return to normal operation while the others 
may lead to shut down of the plant or an accident. Once 
the sequences are described the analyst can rank the 
accidents based on severity of the outcome. The structure 
of the event tree also helps the analyst in specifying where 
additional procedures or safety systems are needed in 
mitigating the accidents or reducing its fi*equency. 

Example: 

In the following figure the initiating event is assigned the 
symbol A, and safety functions the symbols B, C, D. The 
sequences are represented by symbols (A, B, C, D) of the 
events that fail and cause that particular accident. For 
example an error is simply labelled 'A' to interpret the 



15 



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SAFETY 

FUNCTIONS : 



Oxidation reactor 

high temperature 

alarm alerts operator 

at temperature Tl 



Operator 

reestablishes 

cooling water flow 

to oxidation reactor 



Automatic 
shutdown system 
stops reaction at 
temperature T2 



INITIATING EVENT : 

Loss of cooling water 
to oxidation reactor 



Success 



I 



Failure 



SEQUENCE DESCRIPTIONS 



A Safe condition, 
return to normal 
operation 



AC Safe condition, 
process shutdown 



ACQ Unsafe condition, 
runaway reaction, 
operator aware of 
problem 

AB Unstable condition, 
process shutdown 



ABD Unsafe condition, 
runaway reaction, 
operator unaware of 
problem 



'initiating event' occurring with no subsequent failure of 
the safety functions B, C and D. Similarly the sequence 
ACD represents combination of initiating event with 
success of safety function B and failure of safety functions 
C and D. 

A-4 CONSEQUENCE ANALYSIS METHODO- 
LOGIES 

A-4.1 Discharge Rate Models 

Hazardous incidents start with a discharge of a flammable 
or toxic material from its normal containment. Discharge 
can take place from a crack or fracture of process vessels 
or pipe work, an open valve or from an emergency vent. 
The release may be in the form of gas, liquid, or two- 
phase flashing of gas-liquid. 

The discharge rate models provide basic input for the 
following models: 

a) Flash and evaporation model to estimate the fraction 
of a liquid release that forms a cloud for use as 
input to dispersion models, and 

b) Dispersion model to calculate the consequences for 
atmospheric dispersion of the released gas/liquid. 

A-4,2 Flash and Evaporation Models 

The purpose of flash and evaporation model is to estimate 
the total vapour or vapour rate that forms a cloud. 



Superheated liquid stored under pressure at a temperature 
above its normal boiling point, will flash partially or fiiUy 
to vapour when released to the atmospheric pressure. The 
vapour produced may entrain a significant quantity of 
liquids as droplets. The amount of vapour and liquid that 
are produced during flashing of a superheated liquid can 
be calculated from thermodynamics considerations. 
A significant fraction of liquid may remain suspended as 
a fine aerosol. 

The major use of flash and evaporation models is to 
provide an initial prediction of cloud mass — the source 
term for further analysis. 

A-4.3 Dispersion Models 

A-4.3,1 Neutral/ Positively Buoyant Plume and Puff 
Models 

Neutral and positively buoyant plume or puff models are 
used to predict concentration and time profiles of 
flammable or toxic materials downwind of a source based 
on the concept of Gaussian dispersion. Atmospheric 
diffusion is a random mixing process driven by turbulence 
in the atmosphere. Gaussian dispersion models are 
extensively used in the prediction of atmospheric 
dispersion of pollutants. The Gaussian models represent 
the random nature of turbulence. Input requirements for 
Gaussian plume or puff modelling are straightforward. 



16 



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Pasquill and Smith provide description of plume and puff 
discharges [see Foreword (b)] and, that with a risk analysis 
orientation is given by TNO. 

In dispersion model the averaging time for the 
concentration profile is important and generally the 
prediction relate to 1 min averages (equivalent to 1 min 
sampling times). 

A-4.3.2 Dense Gas Dispersion Models 

The importance of dense gas dispersion has become 
recognized for some time and many field experiments have 
confirmed that the mechanisms of dense gas dispersion 
differ markedly from neutrally buoyant clouds. Two 
distinct modelling approaches have been attempted for 
dense gas dispersion: mathematical and physical. 

Detailed descriptions of the mechanisms of dense gas 
dispersion and the specific implementations for a wide 
variety of mathematical models are not given in the 
standard but one may look for in the available guide [see 
Foreword (b)]. The major strength of most of the dense 
gas models is their rigorous inclusion of the important 
mechanisms of gravity slumping, air entrainment, and heat 
transfer processes. 

A-4.4 Fires and Explosions Models 

A-4.4.1 Vapour Cloud Explosions (UVCE) and Flash Fire 

When gaseous flammable material is released a vapour 
cloud forms and if it is ignited before it is diluted below 
its lower explosive limit, a vapour cloud explosion or a 
flash fire will occur. Insignificant level of confinement 
will result in flash fire. The vapour cloud explosion will 
result in overpressures. 

A-4.4.2 Physical Explosion 

When a vessel containing a pressurized gas/liquid ruptures, 
the resulting stored energy is released. This produces a 
Shockwave and accelerated vessel fragments. If the 
contents are flammable then the ignition of the released 
gas could result in fire and explosion. The method 
calculates overpressure. 

A-4.4,3 BLEVE and Fireball 

A Boiling Liquid Expanding Vapour Explosion (BLEVE) 
occurs when there is a sudden loss of containment of a 
pressure vessel containing a superheated liquid or liquified 
gas. It is sudden release of large mass of pressurized 
superheated liquid to atmosphere. The primary cause may 
be external flame impinging on the shell above liquid level 
weakening the vessel and leading to shell rupture. 
Calculations are done for diameter and duration of fu-eball 
and the incident thermal flux. 

A-4.4.4 Pool Fire and Jet Fire 

Pool fires and jet fires are common fire types resulting 



fi-om fires over pools of liquid or Irom pressurized releases 
or gas and/or liquid. They tend to be localised in effect 
and are mainly of concern in establishing potential for 
domino effects and employee safety. Models are available 
to calculate various components — burning rate, pool- 
size, flame height, flame tilt and drag, flame surface emitted 
power, atmospheric transmissivity, thermal flux, etc. 

In jet fire modelling the steps followed for the thermal 
effects are calculation of the estimated discharge rate, total 
heat released, radiant fraction/source view fraction, 
transmissivity and thermal flux and thermal effects. 

A-5 METHODS FOR DETERMINING 
CONSEQUENCE EFFECTS 

Methods are available to assess the consequences of the 
incident outcomes. For assessing the effects on human 
beings, consequences may be expressed in terms of injuries 
and the effects on equipment/property in terms of monetary 
loss. The effect of the consequences for release of toxic 
substances and/or fire can be categorized as: 

a) Damage caused by heat radiation on material 
and people, 

b) Damage caused by explosion on structure 
and people, and 

c) Damage caused by toxic exposure. 

The consequences of an incident outcome are assessed in 
the direct effect model, which predicts the effects on people 
or structures based on predetermined criteria. The method 
increasingly used for probability of personal injury or 
damage is given in Probit analysis. 

The Probit is a random variable with a mean 5 and variance 
1 and the probability (range 0-1) is generally replaced in 
Probit work by a percentage (range 0- 1 00) and the general 
simplified form of Probit function is: 

Px = a + b\x\V 

Where Probit Px is a measure of percentage of variable 
resource, which sustains injury or damage and variable V 
is a measure intensity of causative factor which harms the 
vulnerable resource. 

The causative factor V\ 

a) for fire is thermal intensity and time, 

b) for explosion is overpressure, and 

c) for toxic gas release is toxic dose. 

The constants a and b are calculated from the experimental 
data, which are also available in methods for determinafion 
of possible damage to people and objects resulting from 
release of hazardous materials [see Foreword (01- The 
percentage of fatality with the Probit value {Px) calculated 



17 



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from the equation can be obtained using the chart and table 
given in the methods for determination of possible damage 
[see Foreword (f)]. 

A-5.1 Effect of Fire 

The effect of fire on a human beings is in the form of 
bums. There are three categories of bums such as 'first 
degree', 'second degree' and 'third degree' bum. Duration 
of exposure, escape time, clothing and other enclosures 
play active role while calculating the effect of fire, 
however, the primary considerations are duration of 
exposure and thermal intensity level. 

The heat radiation levels of interest are: 

a) 4 kW/m^: Causes pain if unable to reach cover 
within 20 s, 

b) 4.7 kW/m^: Accepted value to represent injury, 

c) 10 kW/m^: Second degree bum after 25 s, 

d) 12.5 kW/m^: Minimum energy required for melting 
of plastic, 

e) 25 kw/m^: Minimum energy required to ignite 
wood, 

f) 37.5 kW/m^: Sufficient to cause damage to the 
equipment, 

g) 125 KJ/m^: causing first degree bum, 

h) 250 KJ/m^: causing second degree bum, and 
j) 375 KJ/m^: causing third degree bum. 

The thermal effect can be calculated with the help of Probit 
equation for which constants a and b are available. The 
thermal intensity and duration of exposure gives the value 
of K The general equation for the Probit function is: 

FT = a+ b\n tV^, t is duration of exposure and / is thermal 
intensity. 

A-5.2 Effect of Explosion 

The effect of overpressure on human beings is twofold: 

a) Direct effect of overpressure on human organs, and 

b) Effect of debris from stmcture damage affecting 
human. 

Direct ejfect of overpressure on human organ: When the 
pressure change is sudden, a pressure difference arises 
which can lead to damage of some organs. Extent of 
damage varies with the overpressure along with factors 
such as position of the person, protection inside a shelter, 
body weight as well as duration of overpressure. The 
organs prone to get affected by overpressure are ear drum 
and lung. 

Effect of overpressure on stmcture/effect of debris from 
structure damage affecting human: The overpressure 



duration is important for determining the effects on 
structures. The positive pressure phase can last for 10 to 
250 milliseconds. The same overpressure can have 
markedly different effect depending on duration. 
The explosion overpressures of interest are: 

a) 1 .7 bar: Bursting of lung, 

b) 0.3 bar: Major damage to plant equipment stmcture, 

c) 0.2 bar: Minor damage to steel frames, 

d) 0.1 bar: Repairable damage to plant equipment and 
stmcture, 

e) 0.07 bar: Shattering of glass, and 

f) 0.01 bar: Crack in glass. 

The Probit equation can be applied for calculating the 
percentage of damage to stmcture or human beings, the 
constants a and b being available for various types of 
stmctures and the causative factor K depending on the peak 
overpressure, P^. The Probit equation for the overpressure 
is: 

P,= a-^b\T\{Ps) 

A-5.3 Toxic Effect 

The critical toxicity values which should be considered 
for evaluating effect on humans in the event of release of 
chemicals are: 

a) Permissible exposure limits. 

b) Emergency response planning guidelines. 

c) Lethal dose levels. 

A-5.3. 1 Threshold Limit Values {TLV) — Short Term 
Exposure Limit Values (STEL) 

These are the limits on exposure excursions lasting up to 
15 min and should not be used to evaluate the toxic 
potential or exposure lasting up to 30 min. TLV-STEL 
limits are used in evolving measures to protect workers 
from acute effects such as irritation and narcosis resulting 
from exposure to chemicals. Use of STEL may be 
considered if the study is based on injury. 

A-5,3.2 Immediately Dangerous to Life and Death (IDLH) 

The maximum air borne concentration of a substance to 
which a worker is exposed for as long as 30 min and still 
be able to escape without loss of life or irreversible organ 
system damage. IDLH values also take into consideration 
acute toxic reaction, such as severe eye irritation that could 
hinder escape. 

A-5.3.3 Emergency Exposure Guidance Levels (EEGL) 

EEGL is defined as an amount of gas, vapour and aerosol 
that is judged to be acceptable and that will allow exposed 
individuals to perform specific task during emergency 
conditions lasting from 1 to 24 h. 



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18 15656:2006 



A-5.3.4 Short Term Public Emergency Guidance Levels 
(SPEGL) 

These are defined as the acceptable concentration for 
exposures of members of general public. SPEGLs are 
generally set at 10 - 50 percent of EEGL. 

Substances for which IDLH values are unavailable an 
estimated level of concern can be estimated for median 
lethal concentration (LC50) or median lethal dose (LD50) 
levels reported for mammalian species. The LC50 and LD50 
are concentrations or the dose that kill 50 percent of the 
exposed laboratory animals in controlled experiments. 
Lowest reported lethal concentration (LCLO) or lethal 
dose level(LDLO) can also be used as levels of concern. 

Probit equations estimate the injury or mortality rate with 
inputs at two levels: 



a) Predictions of toxic gas concentration and duration 
of exposure. 

b) Toxic criteria for specific health effects for 
particular toxic gas. 

The causative factor K, depends on the above two factors. 
The concentration and exposure time can be estimated 
using dispersion models: 

P^-a + b ln(C"/c) 
where 

C = concentration in ppm by volume, in ppm; 

/^ = exposure time in min; and 

n ^ characteristic constant for that chemical. 



ANNEX B 
{Clause 8) 

FORMAT FOR RISK ANALYSIS REPORT 



B-1 GENERAL 

a) Executive summary, 

b) Introduction, 

c) Objective and scope, 

d) System description, and 

e) Methodology adopted. 

B.2 HAZARD IDENTIFICATION 

a) Hazard Identification methods used and the basis 
for the selection of the methods, 

b) Credible accident sources/worst case scenarios, 

c) Source characteristics, and 

d) Methodology for hazard identification, namely, 
HAZOP and worksheets for identified units, 

B-3 CONSEQUENCE MODELLING 

Result interpretation based on consequence modelling with 



damage contours clearly drawn to scale on site/plot plan 
indicating the population affected. 

B-3.1 Accident Frequency Estimation 

a) System boundaries; 

b) Specific assumption, basic 'frequency data' used 
and its sources; and 

c) Calculated frequency of occurrence of the worst 
accident. 

B-4 DETERMINATION OF PLANT RISK 
Risk criteria. 

B-S LIMITATIONS 

Summary of analytical method, its assumptions and 
limitations. 

B-6 RECOMMENDATIONS 



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IS 15656 : 2006 



ANNEX C 
(Clause 6.1.3) 

PASQUILL-GIFFORD STABILITY CLASSES 

C-1 Insolation category is determined from the table below: 



Surface 

Wind 

Speed, m/s 


Daytime insolataon 


Night Time Conditions 


Anytime 


Strong 


Moderate 


Slight 


Thin 

Overcast of 

> 4/8 low 

cloud 


>3/8 
cloudiness 


Heavy 
overcast 


<2 


A 


A-B 


B 


F 


F 


D 


2-3 


A-B 


B 


C 


E 


F 


D 


3-4 


B 


B-C 


C 


D 


E 


D 


4-6 


C 


C-D 


D 


D 


D 


D 


>6 


C 


D 


D 


D 


D 


D 



NOTES 

A : Extremely unstable conditions. 
B : Moderately unstable conditions. 
C : Slightly unstable conditions. 



D : Neutral conditions. 

E ; Slightly stable conditions. 

F : Moderately stable conditions. 



ANNEX D 
(Clause 6 A 3) 

TERRAIN CHARACTERISTICS PARAMETERS 



Terrain 
Classification 


Terrain Descriptioti 


Surface Roughness 
Zo Meters 


Highly urban 


Centres of cities with tall buildings, very hilly or mountainous area 


3-10 


Urban area 


Centres of towns, villages, fairly level wooded country 


1-3 


Residential area 


Area with dense but low buildings, wooded area, industrial 
site without large obstacles 


1 


Large refineries 


Distillation columns and all other equipment pieces 


1 


Small refineries 


Smaller equipment, over a smaller area 


0.5 


Cultivated land 


Open area with great overgrowth, scattered houses 


0.3 


Flat land 


Few trees, long grass, fairly level grass plains 


0.1 


Open water 


Large expanses of water, desert flats 


0.001 


Sea 


Calm open sea, snow covered flat, rolling land 


0.000 1 



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IS 15656 : 2006 



ANNEX E 
{Clause l.S) 

RISK CRITERIA IN SOME COUNTRIES 



Authority and Application 


Maximum Tolerable Risk 
(Per Year) 


Negligible Risk 
(Per Year) 


VROM, The Netherlands (New) 


l.OE-6 


l.OE -8 


VROM, The Netherlands (existing) 


l.OE-5 


l.OE- 8 


HSE, UK (existing hazardous industry) 


l.OE-4 


l.OE-6 


HSE, UK (New nuclear power station) 


l.OE-5 


l.OE-6 


HSE, UK (Substance transport) 


l.OE-4 


l.OE-6 


HSE, UK (New housing near plants) 


3 X l.OE - 6 


3x l.OE -7 


Hong Kong Government (New plants) 


l.OE-5 


Not used 



21 



18 15656:2006 



ANNEX F 
{Clause 6) 

FLOW CHART FOR CONSEQUENCE ANALYSIS 



SELECTION OF MATERIAL RELEASE 

* RUPTURE / BREAK IN PIPELINE 

* HOLE IN TANK OR PIPELINE 

* RUNAWAY REACTION 

* FIRE EXTERNAL TO VESSELS 

* OTHERS 



SELECTION OF SOURCE MODELS TO 
DESCRIBE RELEASE INCIDENT 

* TOTAL QUANTITY RELEASED 

* RELEASE DURATION 

* RELEASE RATE 

* PHASE OF MATERIAL 



SELECTION OF DISPERSION MODEL 

* NEUTRALLY BUOYANT 

* HEAVIER THAN AIR 

* OTHERS 

RESULTS: 

* DOWNWIND CONCENTRATION 

* AREA AFFECTED 

* DURATION OF DOSE 



TOXIC 




SELECTION OF FIRE AND EXPLOSION 
MODEL 

* TNT EQUIVALENCE 

* MULTI ENERGY 

* FIREBALL 

RESULTS: 

* RADIATION HEAT FLUX 

* BLAST OVER PRESSURE 



EFFECT MODEL 
PROBIT MODEL 



POSSIBLE RESULTS 

* TOXIC RESPONSE 

* NUMBER AFFECTED 

* PROPERTY DAMAGE 



MITIGATION FACTORS 

* ESCAPE / ESCAPE ROUTES 

* EMERGENCY RESPONSE 

* SHELTER IN PLACE, DIKES, 
CONTAINMENTS, ETC. 



22 



IS 15656 : 2006 



ANNEX G 
(Foreword) 

COMMITTEE COMPOSITION 
Occupational Safety and Health and Chemical Hazards Sectional Committee, CHD 8 



Organization 
National Safety Council, Navi Mumbai 
Confederation of Indian Industries, New Delhi 

Indian Chemical Manufacturers Association, Mumbai 

Airport Authority of India, New Delhi 

Atomic Energy Regulatory Board, Mumbai 
Bhabha Atomic Research Centre, Mumbai 

Central Boiler Board, New Delhi 
Central Leather Research Institute, Chennai 
Central Mining Research Institute, Dhanbad 
Central Warehousing Corporation, New Delhi 
Century Rayon, Thane 

Consumer Education & Research Centre, Ahmedabad 

Department of Explosives, Nagpur 
Department of Space (ISRO), Sriharikota 

Department of Industrial Policy and Promotion, New Delhi 
Directorate General Factory Advice Service & Labour Institute, Mumbai 

Directorate General of Health Services, New Delhi 
Directorate General of Mines Safety, Dhanbad 

Directorate of Industrial Safety and Health, Mumbai 
Directorate of Standardization, Ministry of Defence, New Delhi 

Employees State Insurance Corporation, New Delhi 
Hindustan Aeronautics Ltd, Bangalore 
Hindustan Lever Ltd, Mumbai 

Indian Association of Occupational Health, Bangalore 



Representative{s) 

Shri K. C. Gupta {Chairman) 

Shri a. K. Ghose 

Shr] Anik Ajmera {Alternate) 

Shri V. N. Das 

Shri A. A. Panjwani {Alternate) 

Shr] A. N. Khera 

Shri M. Durairajan {Alternate) 

Shri P. K. Ghosh 

Dr B. N. Rathi 

Shri S. Soundararajan {Alternate) 

Representative 

Shrj G Swaminathan 

Shri J. K. Pandey 

Representative 

Shri H, G. Uttamchandani 

Shri S. K. Mishra {Alternate) 

Dr C. J. Shishoo 

Shri S. Yellore {Alternate) 

Representative 

Shri P. N. Sankaran 

Shri V, K. Srivastava {Alternate) 

Dr D. R. Chawla 

DrA. K. Majumdar 

Shri S. R Rana {Alternate) 

Representative 

Director 

Deputy Director {Alternate) 

Shri V. L. Joshi 

Shri P. S. Ahuja 

Lt-Col Tejinder StNGH {Alternate) 

Representative 

Shri S. V. Suresh 

Shri B, B. Dave 

Shri Aditya Jhavar {Alternate) 

Representative 



23 



IS 15656 : 2006 

Organization 
Indian Institute of Chemical Technology, Hyderabad 
Indian Institute of Safety and Environment, Chennai 

Indian Petrochemical Corporation Ltd, Vadodara 

Indian Toxicology Research Centre, Lucknow 

Ministry of Defence (DGQA), Kanpur 

Ministry of Defence (R&D), Kanpur 

Ministry of Environment & Forest, New Delhi 
Ministry of Home Affairs, New Delhi 

National Institute of Occupational Health, Ahmedabad 

National Safety Council, Navi Mumbai 

NOCIL, Mumbai 

Office of the Development Commissioner (SSI), New Delhi 

Oil Industry Safety Directorate (Ministry of Petroleum & Natural Gas), 
Delhi 

Ordnance Factory Board, Kolkata 

Safety Appliances Manufacturers*Association, Mumbai 

SIEL Chemical Complex, New Delhi 

Southern Petrochemical Industries Corporation Ltd, Chennai 

Steel Authority of India Ltd, Ranchi 

TataAlG Risk Management Services Ltd, Mumbai 

BIS Directorate General 



Representative{s) 

Shri S. Venkateswara Rao 

Dr M. Rajendran 

Dr G Venkatarathnam {Alternate) 

Shr] p. Vuayraghavan 

Shri M. R. Patel {Alternate) 

Dr Virendra Mishra 

Dr V. P. Sharma {Alternate) 

Shri M. S. Sultanea 

Shr] SujiT Ghosh {Alternate) 

DrA. K. Saxena 

Dr Rajindra Singh {Alternate) 

Representative 

Shri Om Prakash 

Shri D. K. Shami {Alternate) 

Dr H. R. Rajmohan 

DrA. K. Mukherjee {Alternate) 

Shri R M. Rao 

Shri D. Biswas {Alternate) 

Dr B, V. Bapat 

Shr] V. R. Narla {Alternate) 

Shri Mathura Prasad 

Shrimati Sunita Kumar {Alternate) 

Shri S, K. Chakrabarti 

Shri V. K. Srivastava {Alternate) 

Dr D, S. S. Ganguly 

Shri R. Srinivasan {Alternate) 

Shri M. Kant 

Shri Kirit Maru {Alternate) 

Shri Rajeev Marwah 

Shri Navdeep Singh Birdie {Alternate) 

Shri V Jayaraman 

Shri S. Muruganandam {Alternate) 

Shri V. K. Jain 

Shri Urmish D. Shah 

Dr U. C. Srivastava, Scientist 'F' & Head (Chem) 
[Representing Director General {Ex-officio)] 



Member Secretary 

Shri V. K. Diundi 

Director (CHD), BIS 



24 



IS 15656 : 2006 



Occupational Safety and Health 

Organization 
National Safety Council, Navi Mumbai 
SM India Limited, Bangalore 

Indian Chemical Manufacturers Association, Mumbai 

Airport Authority of India, New Delhi 
Atomic Energy Regulatory Board, Mumbai 
Bhabha Atomic Research Centre, Mumbai 

Central Food & Technological Research Institute, Mysore 
Central Mining Research Institute (CSIR), Dhanbad 
Centre for Fire, Explosives & Environment Safety, Delhi 
Department of Defence Production (DGQA), New Delhi 

Directorate General Factory Advice Services & Labour Institute, Mumbai 

Indian Telephone Industries Ltd, Bangalore 

Industrial Toxicological Research Centre, Lucknow 

ISRO, Shriharikota 

Joseph Leslie & Co, Mumbai 

Joseph Leslie Drager Manufacturing Pvt Ltd, New Delhi 

National Institute of Occupational Health, Ahmedabad 

Oil Industry Safety Directorate, New Delhi 
PN Safetech Private Limited, Lucknow 

Reliance Industries Limited, Mumbai 

Safety Appliances Manufacturers Association, Mumbai 

Standing Fire Advisory Council, New Delhi 

Steel Authority of India, Ranchi 

The Chief Controller of Explosives, Nagpur 

Vishvesvara Enterprises, Navi Mumbai 

Voltech (India), Delhi 



Subcommittee, CHD 8 : 1 

Representative{s) 

Shri p. M. Rao {Convener) 

Shrj Abhueet Arun Saltngikar 
Shri Vfren Shah {Alternate) 

Dr M.S. Ray 

Dr S. H. Namdas {Alternate) 

Shri H. S. Rawat 

Shri V. V. Pande 

Dr D. K. Ghosh 

Shri S. D. Barambe {Alternate) 

Representative 

Shri J. K. Pandey 

Representative 

Shri M. S. Sultania 

Shri B. Ghosh {Alternate) 

Dr A. K. Majumdar 

Shrf S. R Ran a {Alternate) 

Shri P. Jayaprakash 

Shri C. Mahalingam {Alternate) 

Dr a. K. Srivastava 

Dr S. K. Rastogi {Alternate) 

Shri P S. Sastry 

Shri K. Vishwanathan {Alternate) 

Shri Vinod Bamanfya 

Shri S ameer Dange {Alternate) 

Shri Cyril Pereira 

Shri Hirendra Chatterjee {Alternate) 

Dr H. R. Rajmohan 

Dr a. K. Mukerjee {Alternate) 

Representative 

Shri Rajesh Nigam 

Shri Anil Kumar Srivastava {Alternate) 

Shri N. K. Valecha 

Shri S. G. Patel {Alternate) 

Shri M. Kant 

Shri Kirit Maru {Alternate) 

Shri Om Prakash 

Shri D. K. Shami {Alternate) 

Shri V K. Jain 

Representative 

Shri Mahesh Kudav 

Shri Ravi Shinde {Alternate) 

Shri Pawan Kumar Pahuja 

Shri Naresh Kumar Pahuja (Alternate) 



25